Since you can attack the goblins in the single player missions, I think they should be able to attack you occasionally and give you a shield if they beat you. They can steal resources from you but not as much as you would lose in a typical defense. Also if they lose the attack you should get some trophies. They would get better at attacking you the more you upgrade but still it will allow you to have a shield without losing much or you could get easy trophies. They would not attack you very often but maybe once a month on average.
